Objective-C blocks should use the generated Dart class rather than ffi.Pointer<ObjCObject>

[brianquinlan]

Currently a method like:

- (void)readDataOfMinLength:(NSUInteger)minBytes 
                  maxLength:(NSUInteger)maxBytes 
                    timeout:(NSTimeInterval)timeout 
          completionHandler:(void (^)(NSData *data, BOOL atEOF, NSError *error))completionHandler;

Will generate a block like:

class ObjCBlock40 extends _ObjCBlockBase {
...
  void call(
      ffi.Pointer<ObjCObject> arg0, bool arg1, ffi.Pointer<ObjCObject> arg2) {
...

Instead, call should have a signature like:

class ObjCBlock40 extends _ObjCBlockBase {
...
  void call(
      NSData data, bool atEOF, NSError error) {
...

Using the correct argument names is for bonus points ;-)